Here is why I think that is the most likely answer. The cover was sent by surface, not airmail, from Los Angeles to Alameda, a distance of 370 miles. It has a cancellation time of 6 p.m. In that era, the most likely method of transport would have been by train.

And, yes, there was a Southern Pacific run that was an overnight service between Los Angeles and San Francisco/Oakland called the Owl Limited, and it was known to have been used for RPO service.

The Owl Limited began service in 1898, and was discontinued in the 1960s.

The endorsement “OWL MAIL” on letters mailed late in the day would alert mail clerks to punch the letter for transport on the fastest overnight service to Oakland. From there it was a short distance to Alameda.